RFID tags are commonly used to identify persons or goods. RFID tags are usually designed to be compact and not require a battery. Due to these design constraints, RFID tags generally do not have sophisticated memory or processing capability. As a result, nearly all of the processing, management, and power functionality must be provided by the RFID reader. This can be overly burdensome on the reader, particularly if a large number of tags are within range of the reader, which occurs frequently in commercial applications. The reader must handle the volume of communications generated by the tags, organize and identify the tags, and send communications to specific tags. Additionally, if more than one tag responds simultaneously to a query, they will interfere with each other. For this reason, tag identification is an important reader function.
RFID readers use a method called singulation to identify specific tags within a group of tags within range. The singulation protocol or anti-collision protocol prevents tags from interfering with each other, allowing the reader to read data from each tag.
EPC Class 1, Generation 2 (Gen2 protocol) RFID devices use an algorithm similar to slotted ALOHA, called Q-algorithm for singulation. The Q-algorithm is illustrated in FIG. 1. In the Q-algorithm, a Q-parameter is created and all tags are instructed to create a random number for the counter, between 0 and 2^Q−1. The reader then instructs each tag to respond only if its random number is zero, otherwise to decrement its random number by one for the next round. The critical part of the algorithm is initial value of Q. If Q is too large, time will be wasted while tags decrement their random numbers to zero. If it is too small, some tags may guess the same random number, will collide, and will need to be re-identified. In either case, singulation time can be long if the number of tags is very different from 2^Q. All current approaches to determining Q are blind adaptive methods which include some initial guess of Q and its further refinement.
After singulation is finished, tags can be addressed individually (by their EPC IDs) and told to stay quiet (for this purpose, special bits called session inventory flags are used). However, these flags expire (typically, in several minutes) and reset to defaults due to discharge of internal tag capacitors.
There remains a need in the art for a reliably faster and more permanent method of identifying RFID tags.